Polyurethane elastomers are generally well known for use in making shaped articles of manufacture. Methods of making polyurethane elastomers are disclosed for example in U.S. Pat. Nos. 2,969,386 and 4,476,292. Such shaped articles of manufacture include tough wear-resistant parts, foundry patterns, core boxes, impact pads for railroad draft gears, dye pads for metal forming punch presses and a variety of shaped mechanical rubber-like components such as shock mitigation pads.
Another important group of shaped articles made from polyurethane elastomers includes electrically biasable parts for use in electrophotographic copying equipment, such as transfer rollers and transfer belts. Such transfer rollers and belts cooperate electrically with a photoconductive surface to establish a directional force field between the transfer roller or belt and the photoconductive surface. For indirect transfer, the direction of the force field assists in moving a toner-developed photoconductive image from the photoconductive surface to the transfer roller or belt. Subsequently the direction of the force field is reversed so that the image now residing on the transfer roller or belt can be transferred to a more permanent surface such as paper. Indirect transfer is described in U.S. Pat. No. 5,571,457 and also U.S. Pat. No. 5,212,032, the teachings of which are incorporated herein by reference. Direct transfer methods as known in the art can also be employed to transfer toner from the photoconductive surface directly to the permanent surface, i.e., paper.
Transfer rollers, belts and other electrically biased shaped articles used in electrophotographic equipment and their use in an electrophotographic process are further described in detail in U.S. Pat. Nos. 3,959,573, 3,959,574, and 5,897,247, the teachings of which are incorporated herein by reference.
U.S. Pat. No. 3,959,574 describes transfer rollers and belts made from polyurethane elastomers wherein the polyurethane elastomer contains certain additives designed to establish a certain level of resistivity in the elastomers. The additives according to the patent must be soluble in or dispersible in the polyurethane elastomer. The additives are worked into the polyurethane by direct melting of the additive into the polyurethane or incorporating a solution or dispersion of the additive into the polyurethane. The problem is that over time the additives disclosed in U.S. Pat. No. 3,959,574, for various reasons, are leached out of the elastomer. This results in an undesirable decline in the level of conductivity (with a corresponding rise in resistivity) in the polyurethane elastomer, thereby leading to poor copy quality.
Effective transfer of an unfused toned image from a photoconductor surface to a receiver, such as paper, using biased direct or indirect transfer techniques generally requires a relatively constant electric field, and therefore constant current flow, through the biased transfer roller member onto the surface of the receiver. Constant current flow through the transfer roller requires that the bulk resistivity of the roller member, and in particular the semi-conductive polyurethane elastomer, should ideally remain fairly constant regardless of the total amount of charge transferred over the life of the roller member. Typically, however, in actual practice the roller member bulk resistivity increases during use of the member, i.e., its conductivity decreases, which effectively reduces the image transfer latitude with respect to various image receivers and operating environments. Once the roller member resistivity exceeds a critical value, the transfer of images to the receiver is impaired and results in poor copy quality, thereby requiring replacement of the roller.
U.S. Pat. Nos. 4,729,925 and 4,762,941 to Chen et al. disclose certain polyurethane elastomers comprised of charge-control agents for use in making electrically biasable parts in electrophotographic copying equipment. The charge-control agent is said to be added to improve the ability of the polyurethane elastomers to retain pre-established levels of resistivity. The polyurethane is said to be made from a polyisocyanate prepolymer formed with an excess amount of an isocyanate compound. While the resulting polyurethane elastomers can exhibit improved resistance to undesirable increases in resistivity during use, such elastomers can also in some instances lead to initial operational problems and poor copy quality due to an unnecessarily small initial resistivity level. Further, such charge-control agents can be rather expensive to produce and/or obtain commercially, so it would also be desirable to reduce the amount of charge-control agents that are used so as to reduce cost. While such electrically biasable parts exhibit resistance to degradation during use and therefore have a longer useful life, the resistance of the polyurethane material can still increase during use to unacceptable levels.
As can be seen, it would be desirable to develop improved polyurethane elastomers which are capable of exhibiting pre-established levels of resistivity within acceptable levels, both initially and over time during use, so that the performance of electrically biasable parts made with such elastomers can be improved over their useful lifecycle and costs associated therewith are reduced.